1. Field of the Invention
This invention relates to the treatment of formations surrounding oil wells, gas wells, injection wells and similar boreholes by the injection of a treatment fluid. In certain aspects it relates to acidizing, fracturing, or plugging the formation that is being treated.
2. Description of the Prior Art
Many techniques can be broadly described as well treatment techniques. Basically, all well treatments involve the injection of a fluid into an oil or gas well to either stimulate production from the well or to impart certain desired properties to the formation surrounding the well. Probably, the most widely used stimulation technique is hydraulic fracturing where a fracturing fluid is injected into a well under pressure to propagate a fracture adjacent the well. Another widely used stimulation technique is acidizing in which an acidic fluid is introduced into the formation to dissolve formation rock. One acidizing method is known as matrix acidizing whereby an acid is injected into the formation to etch into the pore spaces and fissures naturally present in the formation. Acidizing can also be used in conjunction with hydraulic fracturing whereby the acid reacts on the conductive channels formed by the fracturing operation.
Occasionally, well treatments are designed to achieve the opposite of stimulation; namely, plugging or sealing off a formation to prevent or inhibit the flow of fluids. For example, if a certain zone within an oil or gas formation is producing undesirable quantities of water or brine, it may be necessary to plug the zone by introducing a well treatment fluid carrying a plugging agent which will effectively seal off flow from the zone. In some instances it is desirable to temporarily plug off a zone of a formation so that a following well treatment such as acidizing, can be selectively diverted into another zone of the formation. When the follow-up well treatment is completed, the temporary plugging materials are removed to restore flow to the zone.
For each of the well treatment methods described above, well treatment fluids of various compositions have been designed to enhance the results of the particular treatment. For example, a fracturing fluid should have a sufficiently high viscosity in order to propagate a wide and long fracture in a formation and to transport a large quantity of proppant into the fractures. Currently, oil-in-water emulsions are widely-used, high viscosity, fracturing fluids which contain a major proportion of an oil internal phase dispersed in a minor proportion of a water external phase. Recent developments in the fracturing fluid art pertaining to stabilized oil-in-water emulsions are described in U.S. Pat. Nos. 3,710,865 (Kiel) and 3,977,472 (Graham et al). However, one disadvantage of most oil-in-water emulsion fracturing fluids is that they contain large quantities of oil. As oil prices continue to escalate, the cost of the oil internal emulsions proportionately increases. Simply lowering the concentration of the oil phase is not an acceptable approach since this produces a fluid having insufficient viscosity unless large quantities of polymers are added to the aqueous phase.
With regard to acidizing, treating fluids are preferably designed to achieve delayed acidization. In an acid fracturing treatment, the acid should not attack wellbore tubulars or be rapidly consumed in the area of the formation immediately adjacent the wellbore. Instead, the acid should penetrate deep into the formation where it can achieve a high degree of well stimulation. In a matrix acidization of a carbonate formation, it is undesirable to have a fluid which will promote the formation of wormholes which are short and wide and which do not penetrate deeply into the formation. Instead it is desirable to have long and narrow wormholes which enter the least porous channels where stimulation is necessary. For both acid fracturing and matrix acidizing of carbonate formations, oil internal--acid external emulsions are frequently used to achieve the necessary penetration of acid into the formation. Such emulsion systems are more fully described in U.S. Pat. No. 3,799,266 (Kiel). However, the acid external system has the previously-mentioned disadvantage of containing a high percentage of an expensive oil phase.
Oil-in-water emulsions have also been used as the treating fluids in other types of well treatments such as formation plugging. An oil-in-water emulsion, because of its high viscosity will, by itself, tend to seal off a formation until it is subsequently broken. Secondly, the emulsion serves as an excellent carrier fluid for particulate materials such as diverting and plugging agents. However, once again the logical selection of a highly viscous oil-in-water emulsion is hampered by cost considerations which must account for the expense of using a high percentage oil emulsion.
One approach to overcoming the increasing cost of oil in emulsions used for hydraulic fracturing, acidizing and other well treatment applications, has been to use oil external emulsions. Oil external emulsions generally contain only about 10 to 30 volume percent oil as opposed to the 60 to 80 volume percent typically present in oil internal emulsions. However, the major disadvantage of oil external emulsions which severely limits their use is the increased frictional resistance to flow down well tubulars. U.S. Pat. No. 3,378,074 (Kiel) discloses a technique whereby a lubricating ring of water is used to surround a viscous fracturing fluid, such as an oil external emulsion, as it enters the wellhead. The water ring forms a physical barrier which prevents the viscous fluid from contacting the well tubulars, thereby lowering the friction pressure drop considerably. However, this technique may require the use of special wellhead equipment and fittings to create the conditions necessary to form a film of water between the viscous emulsion and the tubular wall.